1. Field of the Invention
The present invention relates to a step-down switching regulator, and particularly to a control technique for a synchronous rectifier switching regulator.
2. Description of the Related Art
In recent years, microprocessors for providing digital signal processing are mounted in various electronic devices such as cellular phones, PDAs (Personal Digital Assistants), notebook-sized personal computers, etc. The power supply voltage necessary for driving such a microprocessor is being reduced as the fine semiconductor manufacturing process is being improved. For example, a microprocessor is known which operates at a low voltage of 1.5 V or less.
A battery such as a lithium ion battery or the like is mounted on such electronic devices as a power supply. The lithium ion battery outputs voltage of around 3 V to 4V. Such an arrangement, in which the output voltage is directly supplied to the microprocessor, leads to wasteful power consumption, and accordingly, in general, after the battery voltage is stepped down using a step-down switching regulator, a series regulator, or the like, the constant voltage thus stepped down is supplied to the microprocessor.
Two types of step-down switching regulators are known. One is a switching regulator using a rectifier diode (which will be referred to as a “rectifier diode switching regulator” hereafter). The other is a switching regulator using a rectifier transistor instead of the rectifier diode (which will be referred to as a “synchronous rectifier switching regulator” hereafter). The former type has the advantage of exhibiting high efficiency when a load current is low. However, such an arrangement requires a diode, in addition to an inductor and a capacitor, in the form of external components to a control circuit, leading to a large circuit area. On the other hand, the latter type provides poor efficiency when a low current is supplied to the load, as compared with the former type. However, with such an arrangement, a transistor is employed instead of a diode, which allows the control circuit to be integrated in the form of an LSI. This offers a small circuit area incorporating peripheral components. There is a demand for reducing the size of electronic devices such as cellular phones. In many cases, a switching regulator using a rectifier transistor (which will be referred to as a “synchronous rectifier switching regulator” hereafter) is employed in such an arrangement in order to satisfy such a demand for a reduced size.
Directing our attention to the microprocessor employed in the aforementioned electronic devices, when the microprocessor operates for performing computation processing, a certain amount of current flows through the microprocessor. On the other hand, when the microprocessor is in the standby state, only a small amount of current flows through the microprocessor. FIG. 6A is a diagram which shows the current waveform with respect to time when the synchronous rectifier switching regulator is connected to a heavy load. FIG. 6B is a diagram which shows the current waveform with respect to time when the synchronous rectifier switching regulator is connected to a light load. In these drawings, IL represents the current that flows through the output inductor (which will also be referred to as the “inductor current IL” hereafter). Iout represents the load current. Here, the load current Iout is obtained by averaging the inductor current IL over time. As shown in FIG. 6A, when the synchronous rectifier switching regulator is connected to a heavy load, the load current Iout is large. Accordingly, the inductor current IL is always positive. Here, the inductor current IL flowing toward the load is positive by definition. On the other hand, let us consider a case in which the synchronous rectifier switching regulator is connected to a light load as shown in FIG. 6B. In this case, reduction of the load current Iout leads to a negative inductor current IL as indicated by the hatched portion in FIG. 6B. That is to say, in this stage, the direction of the inductor current IL reverses. As a result, with such an arrangement employing the synchronous rectification method, when the synchronous rectifier switching regulator is connected to a light load, current flows from the output inductor to the ground through the synchronous rectifier transistor. This current is supplied from the output capacitor, but is not supplied to the load. This leads to wasteful power consumption.
For example, Patent documents 1 through 3 disclose switching regulators each of which has a function of switching rectification methods between the synchronous rectification method and the diode rectification method based upon the load current. In the techniques described in Patent documents 2 and 3, the inductor current IL is monitored. In a case in which the inductor current changes from a positive value to a negative value, the synchronous rectifier transistor is turned off so as to stop the switching operation, thereby improving the efficiency.
[Patent Document 1]
Japanese Patent Application Laid-open No. 2004-32875
[Patent Document 2]
Japanese Patent Application Laid-open No. 2002-252971
[Patent Document 3]
Japanese Patent Application Laid-open No. 2003-319643
The present inventor has studied a switching regulator which compares the detection voltage that corresponds to the output voltage of the switching regulator with two threshold voltages, i.e., a first threshold voltage at a high level and a second threshold voltage at a low level, using a hysteresis comparator, and which drives a switching transistor and a synchronous rectifier transistor based upon the comparison result. As a result, the present inventor has come to recognize the following problems.
In order to reduce the fluctuation range of the output voltage of a switching regulator employing a hysteresis comparator, the difference between the first threshold voltage and the second threshold voltage, i.e., the hysteresis range is preferably set to as small a value as possible. However, in some cases, extreme reduction of the hysteresis range adversely affects the switching control operation. Furthermore, irregularities in the process for manufacturing the hysteresis comparator lead to irregularities in the hysteresis range. Accordingly, extreme reduction of the hysteresis range adversely affects the switching control operation due to irregularities in the process of manufacturing the hysteresis. Giving consideration to such circumstances, there is a need to set the hysteresis range to a predetermined value or higher.
As described in the aforementioned Patent documents, after the switching operation is stopped in the light-load state, the detection voltage gradually drops according to the reduction in the output voltage. Accordingly, when the detection voltage drops to the second threshold voltage of the hysteresis comparator, there is a need to raise the output voltage by restarting the switching operation. However, restarting of the switching operation involves a certain delay after stopping of the switching operation in the light-load state. As a result, with such an arrangement having a function whereby, in a case in which the detection voltage drops to the second threshold voltage, the switching operation is restarted, the detection voltage drops during this delay time. This leads to increased rippling of the output voltage.